1. Field of the Invention
The present invention relates to an image shake correction apparatus capable of preventing a taken image from degrading by correcting an image shake due to a hand-held shake or the like, and to an image pickup apparatus including the image shake correction apparatus.
2. Description of the Related Art
FIG. 16 is a diagram illustrating an outline of an image shake correction apparatus included in a conventional camera. A shake generated in the camera has a total of six degrees of freedom, i.e., a rotational movement with three degrees of freedom which includes pitching A, yawing B, and rolling C movements and a translational movement with three degrees of freedom which includes movements in X axis, Y axis, and Z axis directions. A currently-commercialized image shake correction apparatus normally corrects a shake due to a rotational movement with two degrees of freedom which includes the pitching and yawing movements that exert a large influence on image shake correction.
A shake of the camera is monitored by an angular velocity sensor 130. Normally, as the angular velocity sensor 130, an angular velocity sensor of a piezoelectric vibration type for detecting a Coriolis force generated by a rotation is used. Three detection units are built into the angular velocity sensor 130. The three detection units detect a pitching shake A that is a rotation about a Z axis of FIG. 16, a yawing shake B that is a rotation about a Y axis of FIG. 16, and a rolling shake C that is a rotation about an X axis (optical axis) of FIG. 16.
In performing correction of an image shake due to a hand-held shake or the like, an output from the angular velocity sensor 130 is sent to a lens CPU 106, and a target drive position of a correction lens 101 for image shake correction is calculated. Instruction signals are sent to voltage drivers 161x and 161y in order to drive the correction lens 101 to the target drive position, and the voltage drivers 161x and 161y drive lens drive units 120x and 120y, respectively, according to the instruction signals. A position of the correction lens 101 is monitored by lens position detection units 110x and 110y, and fed back to the lens CPU 106. The lens CPU 106 performs position control for the correction lens 101 based on the target drive position and the position of the correction lens 101. An image shake caused by a hand-held shake or the like can be corrected by thus driving a correction lens according to a shake.
However, in the above-mentioned image shake correction apparatus, detection of a shake such as a hand-held shake is performed only by the angular velocity sensor 130. Therefore, an angle shake (rotation shake) can be monitored, but a shake that causes the optical axis to move up and down and left and right in parallel (hereinafter, referred to as “parallel shake”) cannot be monitored. Accordingly, the image shake correction can be performed only for the shake due to the movement with the two degrees of freedom that includes the pitching and yawing movements.
Here, an image shake caused by the parallel shake is described by taking an example of performing image taking by using a microlens having a focal length of 100 mm. In a case where the lens is used to take an image of scenery at an infinite distance, assuming that the output from the angular velocity sensor is equivalent to 0.8 deg/s, an image plane moving velocity is approximately 1.40 mm/s (=100×sin 0.8) from the focal length. Therefore, when an image is taken at an exposure time of 1/15 s, a shake width of an image plane caused by the angle shake is 93 μm (=1.40 mm/15). Further, an entirety of the camera may perform a parallel movement in a vertical direction at 1.0 mm/s in addition to the above-mentioned angle shake. In such a case, an image taking magnification value R used for taking an image at the infinite distance is substantially zero. Hence, there is no influence of a parallel moving velocity component, and hence an image shake caused by the parallel shake is not generated.
However, the image taking magnification used for taking a close-up image of a flower or the like is extremely large, which does not allow ignoring the influence of the parallel shake. For example, if an image is taken at the same image taking magnification (R=1) with a vertical moving velocity of 1 mm/s, the image plane also has an image moving velocity of 1 mm/s. When an image is taken at the exposure time of 1/15 s, the shake width in the image plane is 67 μm, and hence the image shake caused by the parallel shake can no longer be ignored.
Next, a general method (model or equation) for expressing the movement of an object in a space in terms of physics/engineering is described. Here, for the sake of simplicity, a general model for expressing the movement of an object on a plane is described. In this case, by defining three degrees of freedom of the object, the movement or the position of the object can be uniquely defined.
FIGS. 17A and 17B illustrate a first model for expressing the movement by the translational movement and the rotational movement. In a fixed coordinate system O-XY of a plane with a lateral axis set as the X axis and the axis orthogonal thereto set as the Y axis, as illustrated in FIG. 17A, the position of the object can be defined by specifying the three degrees of freedom including the position X(t) in the X axis direction, the position Y(t) in the Y axis direction, and a rotational angle θ(t) of the object itself. As illustrated in FIG. 17B, the movement (velocity vector) of the object can be expressed by three components including a translational velocity in the X axis direction Vx(t) of a reference point set on the object, a translational velocity in the Y axis direction Vy(t) of the reference point, and a rotational angular velocity θ′(t) about the reference point on the object. The above-mentioned model is most general.
FIG. 18 illustrates a second model for expressing the movement by an instant rotational center and a rotational radius. It is assumed that the object is revolving about an instant rotational center of a given point f(t)=(X(t),Y(t)) at the rotational radius R(t) with a rotation velocity θ′(t) in the fixed coordinate system O-XY of an XY plane at a given instant. In such a manner, the movement in a plane can be expressed by a locus of the instant rotational center f(t) and the rotation velocity θ′(t) at the instant. The above-mentioned model is often used for analysis of a link mechanism of mechanics.
In recent years, Japanese Patent Application Laid-Open No. H07-225405 and Japanese Patent Application Laid-Open No. 2004-295027 disclose cameras for correcting the parallel shake. In Japanese Patent Application Laid-Open No. H07-225405, a camera shake movement in a three-dimensional space may be expressed by the translational movement and the rotational movement based on measured values of three accelerometers and three angular velocity sensors.
Further, in Japanese Patent Application Laid-Open No. 2004-295027, as illustrated in FIG. 2 thereof, a distance n from a focal plane of a rotation center is calculated for a camera shake including the angle shake and the parallel shake. By Equation 1 of Japanese Patent Application Laid-Open No. 2004-295027, an amount of an angle shake generated when the focal plane is set as the rotation center is calculated in the first half, and an amount of the parallel shake generated due to the parallel movement is calculated in the second half. The parallel shake amount in the second half is a correction term considered in terms of the rotation at the position spaced apart from the focal plane by the distance n. As illustrated in FIG. 3 of Japanese Patent Application Laid-Open No. 2004-295027, the position n of the rotation center is obtained by using an instant center, which is a conception often used in mechanics, as the model for expressing the movement in a space. The above-mentioned conception is that the movement in a space can be expressed by continuous rotational movements, in which a rotational movement about a given point at a given radius at the instant is followed by a rotational movement about a subsequent given point at a given radius at the subsequent instant. Therefore, in the model disclosed in Japanese Patent Application Laid-Open No. 2004-295027, the camera shake movement may be expressed as the continuous rotational movements having the instant center.
However, the method disclosed in Japanese Patent Application Laid-Open No. H07-225405 raises a problem that an enormous amount of calculation is necessary to obtain the amount of the shake in the image plane and that an algorithm for the calculation may be extremely complicated. Further, there is no description of calculation for correcting a focus shake. Further, in the model disclosed in Japanese Patent Application Laid-Open No. 2004-295027, as described above, the camera shake movement may be expressed as the continuous rotational movements having the instant center. However, the model and the equation present such a problem as described in the paragraph “0047” of Japanese Patent Application Laid-Open No. 2004-295027. That is, the calculation cannot be performed because the position n of the rotation center is ∞ if F1≈F2 (forces applied to two acceleration sensors). Further, the position n of the rotation center being ∞ indicates that no shake is caused by an angle in a pitching direction or a yawing direction, and such a shake cannot be detected by the angular velocity sensor 30. It is also possible to calculate a correction amount by using outputs from the two acceleration sensors, but its accuracy is low, and an enormous amount of the calculation is necessary. Further, the equation described in Japanese Patent Application Laid-Open No. 2004-295027 is not capable of calculating a focus shake.